1. Field of the Invention
The present invention relates to a semiconductor device and, more specifically, to a semiconductor device for an IC card.
2. Description of the Background Art
Conventionally, a non-contact IC card receiving power supply from a reader/writer and communicating data in a non-contact state has been known. In the non-contact IC card, a high frequency signal received by an antenna is rectified to generate a power supply voltage, and internal circuitry is driven by the power supply voltage.
The conventional non-contact IC card, however, has a problem that operation of the internal circuitry is destabilized as the power supply voltage fluctuates due to increase/decrease of operation current of the internal circuitry. In general, a capacitor charged by the power supply voltage is provided as a method of stabilizing the power supply voltage. It is difficult, however, to provide a capacitor of large capacitance in an IC chip for an IC card.
Accordingly, a method has been proposed in which a variable current source is connected parallel to the internal circuitry, and the current of the variable current source is decreased/increased in response to increase/decrease of the operation current of the internal circuitry so as to make constant the power supply current and to stabilize the power supply voltage. Such a method of stabilizing power supply voltage is disclosed, for example, in Japanese Patent Laying-Open No. 9-258836.
This method, however, has a problem that current consumption of the IC card increases, as it is necessary to keep the power supply current at the maximum value of the operation current of the internal circuitry.
Therefore, an object of the present invention is to provide a semiconductor device in which internal circuitry operates stably and which consumes less current.
The present invention provides a semiconductor device, including: an internal circuitry receiving a driving current from a power supply node and performing a prescribed operation; a current supplying circuit capable of controlling a supply current for supplying current to the power supply node; a current absorbing circuit capable of controlling an absorbing current for absorbing a current from the power supply node; and a control circuit for controlling each of the supply current of the current supplying circuit and the absorbing current of the current absorbing circuit so that the power supply node attains to a predetermined potential. The control circuit once decreases/increases and thereafter increases/decreases the absorbing current and increases/decreases the supply current, in response to increase/decrease of the driving current of the internal circuitry. Therefore, the potential at the power supply node can be stabilized and the operation of the internal circuitry can be stabilized. Further, only such an amount of current that corresponds to increase/decrease of the driving current is required to flow. Therefore, compared with the prior art in which a current equal to the maximum value of the driving current must be constantly kept flowing, current consumption can be reduced.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.